The neural structures which regulate gonadotropin release and some forms of behavior and underto a sexual differentiation process during development which appears to be under the control of androgen. The criteria hitherto used to evaluate some aspects of this process have been unsatisfactory. Territorial marking in the Mongolian gerbil is a sexually differentiated behavior which offers unique advantages for investigating sexual differentiation of the brain. Using this model system as a foundation, studies will be undertaken to (1) compare and contrast characteristics of the sexual differentiation process in the genetic male and androgenized female, (2) delineate the neural sites which are involved in sexual differentiation, (3) compare the sexual differentiation of the behavioral control system with that of the gonadotropin control system and (4) investigate the mechanism of steroid action in the sexual differentiation process. These studies will involve the manipulation of the sexual differentiation process by paranatal systemic or hypothalamic administration of steroids or pharmacological agents and neonatal gonadectomy. The effects of neonatal manipulations will be determined by: (1) measuring behavioral responses to exogenous hormones, (2) measuring feedback responses of gonadotropin release to exogenous steroids determined by radioimmunoassay of plasma LH in adulthood and (3) histological analysis of the morphology of specific hypothalamic structures.